In an image forming apparatus including an image carrying member, such as a photoconductor, image formation is performed with processes such as charging, exposure, development, and transfer performed on the image carrying member as the image carrying member rotates. In such an image forming apparatus, a drive source, such as a motor, is provided to drive the image carrying member to rotate. In general, the number of rotations of the drive source exceeds the number of rotations required for the rotation of the image carrying member. Therefore, reduction gear units have been used that transmit the drive force of the drive source to the image carrying member while reducing the rotation speed of the drive source with the use of a planetary gear speed reduction mechanism.
The planetary gear speed reduction mechanism includes a sun gear, an outer gear arranged coaxially with the sun gear, a plurality of planetary gears meshing with the sun gear and the outer gear, and a planetary carrier rotatably supporting the planetary gears and rotatably supported coaxially with the sun gear and the outer gear. The planetary gear speed reduction mechanism has three elements, i.e., the rotation of the sun gear, the revolution of the planetary gears (i.e., rotation of the planetary carrier), and the rotation of the outer gear, one of which is connected to a fixed member, another one of which is connected to an input member, and the remaining one of which is connected to an output member. It is possible to obtain different speed reduction ratios or set the rotation direction of the output shaft opposite to the rotation direction of the input shaft, depending on the combination of the three elements and the fixed, input, and output members.
In recent years, in image forming apparatuses in particular, polymerized toner has been used in many cases to improve image quality. As a consequence, the load applied to the image carrying member by a cleaning blade and so forth has been increasing. To counteract that effect, it is therefore conceivable to increase the output torque by increasing the speed reduction ratio with the use of a compound planetary gear reduction gear unit including a plurality of stages of planetary gear speed reduction mechanisms having the above-described configuration. It is common, in the compound planetary gear reduction gear unit, to use standardized outer gears, planetary gears, and sun gears at the respective stages to reduce costs.
The compound planetary gear reduction gear unit is, however, subjected to load torque of the load imposed by the cleaning blade and so forth on the rotary image carrying member. The thus-applied load torque is greatest at the planetary gear speed reduction mechanism at the extreme downstream stage in the drive transmission direction (hereinafter referred to as the final stage). As a result, the force generated by the load torque is greatest at the meshing portions of the gears in the planetary gear speed reduction mechanism at the final stage. In particular, the force generated by the load torque is greatest at the meshing portions of the sun gear at the rotation center and the planetary gears.
More specifically, where F represents the force generated by the load torque and applied to the meshing portions of the sun gear and the planetary gears, N represents the load torque, and r represents the pitch diameter of the sun gear, the equation F=N/(r/2) holds. Meanwhile, the outer gear is larger in pitch diameter than the sun gear. Therefore, the force generated by the load toque and applied to the meshing portions of the outer gear and the planetary gears is lower than the force applied to the meshing portions of the sun gear and the planetary gears. The force generated by the load torque is thus greater at the meshing portions of the sun gear and the planetary gears in the planetary gear speed reduction mechanism at the final stage. As a result, the sun gear and the planetary gears tend to wear out prematurely.